The Advanced Graphics Port (AGP) protocol is a design standard for a dedicated communications bus to transmit data to and from a host processor (CPU) and a graphics controller (e.g., SVGA controller or the like). The AGP is based upon the successful PCI (Peripheral Communications Interface) bus protocol, but operates at a higher clock speed (66 MHz versus 33 MHz) and is provided with additional commands for graphics applications.
In the AGP protocol, a 66 MHz input clock source is supplied. Data is transferred on the AGP bus at both the rising edges and falling edges of each clock pulse of the 66 MHz clock source. Therefor a 2.times. clock signal based on the input clock source is required to sample data correctly.
In the prior art, an analog Phase Locked Loop (PLL) may have been considered sufficient for such a task, however such an analog circuit may require extensive circuitry within an integrated circuit. Moreover, such analog circuits may be more sensitive to variations in the production process and may be more difficult to operate, particularly at higher operating frequencies. Moreover, such analog circuits may consume more power than digital circuits.